1. Field of the Invention
The present invention relates to an optical detecting sensor, and more particularly, to a thin film transistor (TFT) type optical detecting sensor that has a sensor TFT, and a switching TFT.
2. Discussion of the Related Art
The TFT type optical detecting sensor detects an image of an object by producing a detecting current in response to light reflected from the object. The sensor is comprised of a light source, a window which transmits light from the light source to the object, a sensor TFT which generates an optical current in accordance to the amount of the light reflected from the object, a storage capacitor which stores charges of the optical current generated in the sensor TFT, and a switching TFT which controls the release of charges of the storage capacitor to an external circuit.
Since the sensor TFT and the switch TFT are comprised of similar elements, in making the detecting sensor, the active layers for the sensor TFT and the switching TFT are preferably deposited and patterned at the same time. The active layer for the sensor TFT should be chosen among the materials that are sensitive to light and active in converting light into current. Also the active layer of the switching TFT should be proper for the switching operation. Thus, to satisfy both conditions, the active layer for the TFTs is preferably an amorphous silicon layer having hydrogen (a-Si:H, herein after referred as “amorphous silicon layer”).
Since the switching TFT is operated by a voltage applied to a gate of the switching TFT, not by the light, the active layer of the switching TFT should be shielded from light, whereas the active layer of the sensor TFT generates optical current by the incident light reflected from the object.
FIG. 1 is a schematic sectional view of a conventional optical detecting sensor, which can explain a manufacturing process.
First, on a transparent and insulating substrate 10, regions for a window, a sensor TFT, a storage capacitor, and a switching TFT should be defined as the areas “D”, “C”, “B” and “A”, respectively, in advance.
A first conducting metal layer is deposited on the substrate 10 and patterned into a sensor gate electrode 11, a first capacitor electrode 13 and a switch gate electrode 15 in the corresponding areas, respectively.
A first insulating layer is deposited on the substrate 10 while covering the patterned conducting metal layer.
An amorphous silicon layer and an n+ amorphous silicon layer are deposited in succession on the first insulating layer to form sensor and switch active layers 19 and 21 and ohmic contact layers 23 and 25 for the corresponding active layers 19 and 21, respectively.
A second conducting metal layer is deposited on the first insulating layer and patterned into sensor source and drain electrodes 27a and 27b, a second capacitor electrode 29 and switch source and drain electrodes 31a and 31b. The second conducting metal layer may be made of a general non-transparent metal or a transparent conducting material, for example, indium tin oxide.
A second insulating layer 33 is deposited on the second conducting metal layer and the exposed portion of the active layers 19 and 21. Then a light shielding layer 35 is formed on the second insulating layer 33 over the switch active layer 21.
The contact portion between switch drain electrode 31b and the switch active layer 21 is shown in detail in FIG. 2, which is an enlarged view of portion “J” of FIG. 1.
The upper portion of the switch active layer indirectly contacts the switch drain electrode 31b through the ohmic contact layer 25 that restricts the hole current flow, whereas the side portion “w” of the switch active layer 21 directly contacts the switch drain electrode 31b. Since there is no ohmic contact layer in the side contact area “w” of the switch active layer 21, the leakage current to the switch drain electrode 31b is bigger than in the upper contact portion of the switch active layer 21, resulting in an adverse affect on the operation characteristics of the switch TFT.
FIG. 3 shows the relationship between gate voltage VG2 for switching TFTs having drain electrodes made of different materials and drain current ID. If the drain electrode is made of transparent conducting material, as illustrated in solid line 36, the leakage current (or off-current) and the on-current are low, since resistance of the transparent conducting material is relatively high, which means that there is a small current gap between negative and positive voltage of gate electrode. If the drain electrode is made of a non-transparent metal, as illustrated in the dot-dash line 34, both the off-current and the on-current are relatively high, also resulting in a small gap between the off-current and on-current.